1. Field of the Invention
The invention relates to the production of improved metal alloy filaments extracted from molten metal alloy sources and quenched on a chill or quench wheel, and in particular, to the production of improved amorphous metal alloy filaments by casting in partial vacuum.
2. Description of the Prior Art
The process of fabricating metal filaments by extracting from a molten metal source and quenching on a quench wheel is often referred to as melt spinning of chill-block spinning. Typically, as taught in U.S. Pat. Nos. 2,825,108, 2,886,866 and 2,899,728, a free jet of molten material is impinged upon a moving quench surface in air, preferably a rotating wheel or continuous belt, and rapidly quenched to produce a product having superior physical properties. Various melt spinning techniques are employed to obtain solid solutions of metals that would normally separate on solidification due to mutual insolubility in the solid state. These techniques have long been employed to produce polycrystalline metal products possessing a very fine grain crystalline structure. More recently, melt spinning has been used to produce glassy or amorphous metal alloy filaments, which require quenching of the melt at a rate such that the particular alloy reaches its characteristic glass transistion temperature before departure from the quench source. Typically, quench rates of about 10.sup.5 .degree. to 10.sup.6 .degree. C./sec must be attained to achieve the desired amorphous structure.
As is well-known, an amorphous material generally characterizes a noncrystalline or glass material, that is, a material substantially lacking any long range order. In distinguishing an amorphous material from a crystalline material, X-ray diffraction measurements are generally suitably employed. Additionally, transmission electron micrography and electron diffraction can be used to distinguish between the amorphous and the crystalline state.
An amorphous metal produces an X-ray diffraction profile in which intensity varies slowly with diffraction angle. Such a profile is qualitatively similar to the diffraction profile of a liquid or ordinary window glass. On the other hand, a crystalline metal produces a diffraction profile in which intensity varies rapidly with diffraction angle.
These amorphous metals exist in a metastable state. Upon heating to a sufficiently high temperature, they crystallize with evolution of a heat of crystallization, and the X-ray diffraction profile changes from one having glassy or amorphous characteristics to one having crystalline characteristics.
It is possible to produce a metal which is totally amorphous or which comprises a two-phase mixture of the amorphous and crystalline state. The term "amorphous metal", as employed herein, refers to a metal which is at least 50% amorphous, and preferably 80% amorphous, but which may have some fraction of the material present as included crystallites.
For the purpose of the invention, the term "filament" is meant to include any slender metallic body whose transverse dimensions are substantially less than its length. These filaments may be ribbon, wire or sheet or may have an irregular cross-section.
Recent modifications in melt spinning processing techniques and apparatus have resulted in improved amorphous metal filaments. See, for example, U.S. Ser. No. 360,888, filed May 16, 1973 by J. R. Bedell, now U.S. Pat. No. 3,862,658, issued Jan. 28, 1975, which discloses extending the retention time of the molten stream of metal on the quench wheel. While such improvements are beneficial, a recurring problem remaining involves dimpling or blister formation on the filament surface which is in contact with the quench wheel.